Method for Substrate Stabilization of Diffusion Aluminide Coated Nickel-Based Superalloys

ABSTRACT

An article and method for stabilization of a nickel-based superalloy coated with a diffusion aluminide coating. The region below the aluminide coating is first carburized to form refractory carbides. The article is cleaned and masked as required so that regions that will not have an aluminide coating are not carburized. After placing the article into a furnace and heating in a non-oxidizing atmosphere to a carburizing temperature, a carburizing gas is introduced, and the near surface region is carburized to a depth of about 100 microns. Refractory carbides are formed in this region. When a diffusion aluminide coating is formed on the article, the refractory elements, being present as refractory carbides, are not available to form detrimental TCP phases.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/359,788, filed Feb. 22, 2006, which is incorporated by reference inits entirety and which claims the benefit of U.S. ProvisionalApplication No. 60/656,691, filed Feb. 26, 2005.

FIELD OF THE INVENTION

The present invention relates to the carburization of nickel-basedsuperalloys, and more particularly, to methods for carburizingnickel-based superalloys that include refractory elements for preventingthe formation of secondary reaction zones.

BACKGROUND OF THE INVENTION

In a gas turbine engine such as used for aircraft applications, air isdrawn into the front of the engine, compressed by a compressor, andmixed with fuel. The compressed mixture is burned in a combustor, andthe hot combustion gases flow through a turbine that turns thecompressor. The hot gases then flow from the rear of the engine.

The turbine includes stationary turbine vanes that deflect the hot gasflow sideways, and turbine blades mounted on a turbine wheel that turnsas a result of the impingement of the hot gas stream. The turbine vanesand blades experience extreme conditions of high temperature, thermalcycling when the engine is turned on and off, oxidation, corrosion, and,in the case of the turbine blades, high stress and fatigue loadings. Thehigher the temperature of the hot combustion gas, the greater theefficiency of the engine. There is therefore an incentive to push thematerials of the engine to ever-higher temperatures and loadings.

Nickel-based superalloys are widely used as the materials ofconstruction of gas turbine blades and vanes. These superalloys containprimarily nickel, and a variety of alloying elements such as cobalt andaluminum, as well as refractory elements such as tantalum, tungsten,chromium, rhenium, hafnium, and others in varying amounts carefullyselected to provide good mechanical properties and physicalcharacteristics over the extremes of operating conditions experienced bythe engine. However, these refractory elements, which provide thenickel-based superalloys with superior mechanical properties, also makesuperalloy articles susceptible to the formation of a secondary reactionzone (“SRZ”) in certain circumstances. In particular, gas turbine alloyairfoils, such as the turbine blade and vanes discussed above, typicallyrequire an aluminide coating treatment as part of a thermal barriercoating system and/or to provide environmental protection. Nickel-basedsuperalloy articles that include refractory elements and which undergoaluminiding treatments are particularly susceptible to formation of anSRZ, wherein an acicular topologically close-packed (TCP) phase forms,such as disclosed in “A New Type of Microstructural Instability inSuperalloys—SRZ,” Superalloys, 1996 by W. S. Walston, J. C. Schaefferand W. H. Murphy, ed. R. D. Kissinger, et al. TMS pp. 9-18. Within theSRZ, the TCP phases are brittle and contain a high percentage ofrefractory elements. In particular, the presence of the brittle phases,the formation of high angle grain boundaries between the SRZ and thealloy, and to a lesser extent, the depletion of the refractory elementsweaken the SRZ, making the SRZ essentially non-load-bearing. Becausethis portion of the article is unable to sustain its share of the load,the applied load is shifted to the remainder of the article, increasingthe stress in this portion of the article and shortening its servicelife.

The problem with refractory elements in nickel-based superalloy articlesforming SRZ is known, having been identified in U.S. Pat. No. 5,334,262,entitled SUBSTRATE STABILIZATION OF DIFFUSION ALUMINIDE COATEDNICKEL-BASE SUPERALLOYS issued Aug. 2, 1994 to Schaeffer and assigned tothe assignee of the present invention. This patent also identifiesforming carbide precipitates which reduce the driving force for theformation of TCP phases within the substrate, a method for avoiding theformation of SRZ, by depositing a layer of carbon on the surface of thesubstrate by chemical vapor deposition and diffusing the carbon onto thesurface. The presence of the carbon allows for the combination of carbonwith the refractory elements to form stable carbides, substantiallyreducing the refractory elements available for the formation of TCPphases. This patent, U.S. Pat. No. 5,334,262 is incorporated in itsentirety herein by reference, forming a part of this specification.

Carbon can be introduced into the nickel-based superalloy article bycarburizing techniques, such as vacuum carburizing. Vacuum carburizingof steel is a well-known technique. U.S. Pat. No. 4,836,864 issued Jun.6, 1989, entitled “Method of Gas Carburizing and Hardening” disclosesgas carburizing and hardening a steel article in a carburizingatmosphere at atmospheric pressure, heating the article in a vacuum fora predetermined period of time and hardening the article. U.S. Pat. No.5,702,540 issued Dec. 30, 1997 entitled “Vacuum Carburizing Method andDevice, and Carburized Products” teaches vacuum carburizing steelworkpieces in a vacuum furnace by introducing acetylene gas into thechamber at a vacuum of 1 kPa or less to produce a hardened and uniformcase depth in the steel article. U.S. Pat. No. 6,187,111 Feb. 13, 2001entitled “Vacuum Carburizing Method” divulges an improved vacuumcarburizing method for steel by heating the steel material to about900-1100° C. and then introducing ethylene gas while maintaining avacuum of 1-10 kPa, thereby eliminating the potentially explosiveacetylene and replacing the expensing vacuum pumps or mechanical boosterpumps required to maintain vacuums at or below 1 kPa.

Of course, it may also be desirable to prevent selected portions of thearticle from being carburized by preventing contact of the surface withcarbon. It is known to mask all or a selected portion of an articlesurface with a cover or coating to prevent it from being carburized.These coatings or covers, also referred to as a maskant, typically areplatings and are usually very effective. These coatings, however, mustbe easy to remove or must be incorporated into the article. Typicalmaskants include nickel plating and copper plating. However, suchplating may be unsuitable for articles that have precise shapes orinclude intricate details, since removing such plating aftercarburization can be difficult or impossible without damaging thearticle. However, a boron glass coating used as a maskant containingmagnesium silicon compounds may be acceptable for use on intricatearticles such as turbine blades, as this material can provide protectionfrom carburization to selected, intricate areas of an airfoil, yet canbe removed without damaging the airfoil. This maskant system isdescribed in U.S. Published Application No. 20020020471 A1, publishedFeb. 21, 2002, and also is incorporated herein by reference.

Coatings typically are formed on the surfaces of the superalloy articlesto protect the article from degradation in harsh, high temperatureenvironments. One type of coating is an aluminide coating. Aluminum isdiffused into the surface of the nickel-based superalloy article to forma nickel-aluminide layer, which then oxidizes to form an aluminum oxidesurface coating during treatment or in service. (Optionally, noblemetals such as platinum may also be diffused into the surface). Thealuminum oxide surface coating renders the coated article more resistantto oxidation and corrosion, desirably without impairing its mechanicalproperties. Aluminide coating of turbine blades and vanes is well knownand widely practiced in the industry, and is described, for example, inU.S. Pat. Nos. 3,415,672 and 3,540,878.

Recently it has been observed that, when some advanced nickel-basedsuperalloys are coated with an aluminide coating and then exposed toservice or simulated-service conditions, a secondary reaction zone (SRZ)forms in the underlying superalloy. This SRZ region is observed at adepth of from about 50 to about 250 micrometers (about 0.002-0.010inches) below the original superalloy surface that has received thealuminide coating. The presence of the SRZ reduces the mechanicalproperties in the affected region, because the material in the SRZappears to be brittle and weak, and forms a high angle grain boundarybetween SRZ and the alloy.

The formation of the SRZ is a major problem in some types of turbinecomponents, because there are cooling channels located about 750micrometers (about 0.030 inches) below the surface of the article.Cooling air is forced through the channels during operation of theengine, to cool the structure. If the SRZ forms in the region betweenthe surface and the cooling channel, it significantly weakens thatregion and can lead to reduced strength and fatigue resistance of thearticle.

While the prior art prevents the formation of the TCP phases that weakenthe SRZ, the prior art relies solely on a diffusion mechanism to diffuseinward the carbon deposited on the surface of the superalloy substrate.While acceptable results can thus be obtained, it is desirable toimprove the methods of deposition to control the depth of carburizationwhile allowing the absorption of carbon into the surface quickly.

SUMMARY OF THE INVENTION

The present invention provides methods for carburizing a nickel-basedsuperalloy that includes refractory elements using alkynes or ethylene(C2H4) as the carburizing agent. In accordance with the presentinvention, a nickel-based superalloy that includes refractory elementsis carburized, under controlled conditions, using alkyne gases, propaneor ethylene gas (C2H4) or combinations thereof as the carburizing agentin order to form stable refractory carbides at a controlled, preselecteddistance below the surface. These stable refractory carbides reduce thedriving force for the formation of TCP phases that would otherwiseproduce a weak SRZ in the controlled, preselected distance at and belowthe substrate surface.

The present invention contemplates cleaning the article surface.Cleaning the article surface entails removing all oxides from thesurface of the substrate and preventing the reformation of oxides fromthe surface that is to be carburized. It is imperative that the surfacethat is to be carburized is free of oxides. Removing oxides can beaccomplished by mechanical or chemical methods which do not damage orotherwise adversely affect the substrate surface. After such cleaning,the surfaces may be cleaned with a suitable solvent, while avoiding theformation of new oxides. While oxides are to be avoided, it may bedesirable to mask portions of the surface in order to prevent theseportions from being carburized. This may be desirable for any one of anumber of reasons, such as portions of the surface may not be exposed toan aluminizing treatment or the stresses in the portion of the surfaceare not determinative of part life in that portion of the article. Inthis event, the portion which should not be carburized is masked. Themasking should prevent carburizing of the area masked, should be stableat the elevated temperatures of operation, and should be easy to removeafter carburizing, or otherwise be capable of being incorporated intothe article.

The cleaned article is then loaded into a furnace suitable forperforming the carburization process while also preventing the formationof oxides. Suitable furnaces include vacuum furnaces or furnaces thatcan maintain a controlled atmosphere. When maintaining a controlledatmosphere, the atmosphere must be non-oxidizing, as oxidation of thearticle surface must be prevented during heat-up to the carburizingtemperature and during carburizing. Once the carburizing temperature isapproached, the carburizing gas, alkyne, propane or ethylene, isintroduced into the furnace. These carburizing gases may be introducedbelow the carburizing temperature with hydrogen or to gradually replacehydrogen, but should not be added at a temperature or in a volume thatwill result in excessive soot formation. The carburizing gas is providedeither on a continuous basis or by a pulse method. Regardless of themethod, the carburizing gas is provided to ensure sufficient carbon ispresent at the surface for desired carburization so that carbides areformed in a layer of sufficient thickness, so that the layer will notform TCP phases after subsequent exposure to aluminum as a result ofaluminizing. The article will thus be free of the SRZ. The duration ofthe carburization process itself is controlled to limit the depth ofcarbide layer formation, since carbide layers that are too thick alsocan adversely affect the mechanical properties of the article. Clearly,over-carburization that produces a layer that is too thick results in asubstrate that is devoid of the beneficial effects of the refractoryelements, as the refractory elements are tied up in the stable carbides.

The carburization process is completed by purging the chamber of thecarburizing gas. This can be accomplished by stopping the flow of thecarburizing gas and introducing an inert gas, nitrogen or hydrogen intothe chamber. This also serves to quickly cool the article. Any maskingthat has been applied may now be removed. The article can now be heattreated to in any way, such as by aging, to cause the precipitation ofdesirable strengthening phases such as gamma prime, if these phases havenot previously been formed. The article can now also be aluminided. Thealuminiding treatment can be accomplished by the addition of an additivealuminide layer. Alternatively, the aluminide may be by a thermallygrown aluminide method, in which the aluminide layer is grown into thetop surface. While the method of aluminiding does not matter, it isimportant that the aluminum from the aluminide process does notpenetrate significantly (a few microns) below the layer of carbides.

An advantage of the present invention is that the process can be carriedout quickly and the carburization depth, the depth of formation of therefractory carbides, can be closely controlled, because the use ofcontrolled flow of the reactive alkyne gases or ethylene provides thenecessary supply of carbon at the surface of the article, increasing thechemical activity of carbon at the surface as compared to prior artmethods of introducing carbon to the surface.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a superalloy article.

FIG. 2 is a schematic sectional view of the article of FIG. 1 takengenerally along line 2-2, illustrating the near-surface region during adiffusional platinum-aluminiding treatment when the treatment of thepresent invention is not used.

FIG. 3 is an enlarged, diagrammatic sectional view of the microstructurenear the surface in the region depicted in FIG. 2.

FIG. 4 is a process flow chart for the treatment of the invention.

FIG. 5 is an enlarged, diagrammatic sectional view of the microstructurenear the surface of an article like that depicted in FIGS. 1-3, exceptthat the process treatment of FIG. 4 has been utilized.

DETAILED DESCRIPTION OF THE INVENTION

The stabilization approach of the invention is used with nickel-basedsuperalloys, in applications such as a jet engine airfoils includingturbine vanes and the illustrated gas turbine blade 10 of FIG. 1. Theblade may be formed of any nickel-based superalloy that has a tendencyto form a surface secondary reaction zone during and after an aluminidetreatment. These superalloys typically include reactive elements such astantalum, tungsten, molybdenum, chromium, rhenium, hafnium, ruthenium,iridium, osmium, and in certain alloys, platinum, palladium and rhodium,and others which are added to provide enhanced mechanical propertiessuch as strength. Examples of such nickel-based superalloys include Rene162, whose composition is disclosed in U.S. Pat. No. 5,151,249incorporated herein by reference and the alloy known as 4EPM-102C whosecomposition is disclosed in Walston et al., Superalloys, TMS,Warrendale. Pa., 15-34, 2004. Rene 162, for example, has a compositionin weight percent of about 12.5 percent cobalt, 4.5 percent chromium,6.25 percent rhenium, 7 percent tantalum, 5.74 percent tungsten, 6.25percent aluminum, 0.15 percent hafnium, 0.5 percent yttrium, minoramounts of other elements, and balance nickel.

The blade 10 includes an airfoil section 12 against which hot combustiongases are directed when the engine operates, and whose surface issubjected to severe oxidation and corrosion attack during service. Theairfoil section 12 is anchored to a turbine disk (not shown) through adovetail or root section 14. In some cases, cooling passages 16 arepresent in the airfoil section 12, through which cool bleed air isforced to remove heat from the blade 10. The blade is normally preparedby a casting and solidification procedure well known to those skilled inthe art, such as investment casting, directional solidification, orsingle crystal growth.

FIGS. 2 and 3 are sections through the blade 10 showing the result of aconventional aluminide treatment without the benefit of any carburizingtreatment, such as the carburizing treatment of the present invention.An aluminum-containing layer 20 is formed on a surface 22 of the airfoilsection 12, which acts as a substrate 24. Optionally, in some cases athin layer 26 of nickel or a noble metal, such as a platinum-containinglayer, may be deposited on the surface 22 prior to deposition of thealuminum-containing layer 20. After application of layer 20 over surface22, the blade 10 is heated to elevated temperature so that there isinterdiffusion between the layer 20 (and optional layer 26) and thesubstrate 24, indicated generally by the arrows 28. The type, amount,and extent of the interdiffusion depends upon a number of factors suchas time, temperature, substrate alloy, and activity of the aluminumsource. Either during or after this process, an upper surface 30 isallowed to oxidize, forming an aluminum oxide layer (not shown). Anoptional ceramic topcoat may be applied over this outer layer.

The acicular TCP phases will vary from alloy to alloy, as the contentand specific refractory elements will vary from alloy to alloy. Thecomposition of the TCP phases in Alloy Rene '162 are discussed in U.S.Pat. No. 5,334,263. The actual chemical composition is not important, asthe phases typically include refractory elements, depleting thesurrounding matrix of these elements and forming the brittle acicularstructure, thereby weakening the material matrix in secondary reactionzone 34 of FIG. 3. FIG. 3 depicts the resulting metallurgicalmicrostructure in a typical refractory element-containing nickel-basedsuperalloy turbine blade that has been aluminized. Two types ofdiffusion zones are produced. A primary diffusion zone 32 containing TCPphases, such as a sigma (Σ) phase, a mu (η) phase or a ρ-phase, eitheralone or in combination, in a beta or gamma prime matrix form just belowthe layer 20. The secondary reaction zone (SRZ) 34 forms between theprimary diffusion zone 32 and the substrate 24. The SRZ 34 has beendetermined to result in reduced mechanical properties of the blade 10,particularly when it occupies a substantial fraction of the materialbelow the surface 22. This situation is exacerbated when there is also asubsurface cooling passage 16 (FIG. 2), which almost invariably ispresent.

The present approach reduces the amount of available refractory elementreactants available to form TCP phases in the near-surface,aluminum-rich regions by utilizing a carburizing process to form stablerefractory carbide compounds in near surface regions 32 and 34 of thearticle within a preselected distance from the surface, while notreducing the refractory element concentration in other regions remotefrom the surface.

A preferred process for reducing the amount of available refractoryelement reactants that would otherwise be available to form the TCPphase is depicted in FIG. 4. A refractory containing nickel-basedsuperalloy article, such as Rene 162, containing at least one elementfrom the group consisting of rhenium, chromium, tantalum, molybdenum,tungsten ruthenium, iridium, osmium, and in certain alloys, platinum,palladium and rhodium, is provided. This alloy would form TCP phasesfollowing the aluminide process, unless processed to avoid such TCPphase formation.

According to the present invention, the article surface is cleaned toremove oxides. This can be achieved by mechanical or chemical means.However, it is preferred that the surface be cleaned using grit blastingtechniques having adequate pressure and grit size to remove the surfaceoxides. Acceptable grit size which do not otherwise alter the surfaceinclude 80-600 mesh grit, and preferably 80-220 mesh grit, at a pressureof 20-90 psi, preferably 40 psi, have been found to be acceptable.Alternatively, vapor honing may be used. Chemical etching is analternate method that is acceptable to remove oxides.

After the surface has been cleaned to remove the oxides, the articlesurface must be carefully protected to prevent the reformation ofoxides. This is probably best accomplished by quickly placing theworkpiece into the working zone of the carburizing unit.

However, if there are portions of the article which are not to becarburized, an optional step of masking should be performed. While anysuitable maskant may be utilized, certain maskants pose particularproblems as they can be difficult to remove after carburizing. Forexample, nickel plating or copper plating, each excellent as a maskant,may be difficult or impossible to remove from an article surface havingprecise shapes or intricate details such as a turbine blade. However, acompound of boron glass containing magnesium-silicon, such as describedin U.S. Patent Application Publication No. 20020020471, published Feb.21, 2002, incorporated herein by reference, exhibits exceptionalcapabilities as a suitable maskant. This maskant is particularlypreferred for use in vacuum carburization processes in which processingis performed at high temperatures. This maskant exhibits the threeprincipal properties of a maskant in this application: it is stable atthe elevated temperatures of carburizing, prevents carburization, andcan be easily removed after carburization.

After any optional maskants are applied and while maintaining thecleanliness of the article surface, the article is placed in a workingzone of a suitable furnace. The furnace must be capable of preventingoxidation of the article surface as the article is heated to thecarburization temperature. Thus vacuum furnaces or a furnace that canmaintain a protective atmosphere are preferred. The atmosphere may be aninert atmosphere or a reducing atmosphere, maintained by theintroduction of an inert gas or hydrogen into the furnace or byachieving a vacuum preferably with a pressure of less than 1 Torr.However, a reducing atmosphere, obtained by introducing at least apartial pressure of hydrogen into the furnace is most preferred. Apartial pressure of hydrogen maintained at about 0.0005-10 Torr ispreferred during this stage, but a partial pressure of 0.05-1.0 Torr ismore preferred. As an option, even when the carburizing is to beperformed in a vacuum furnace, a low partial pressure of hydrogen can beintroduced into the vacuum furnace. Even though the hydrogen isultimately removed as the vacuum is pulled by the vacuum pump, thereducing atmosphere assists in preventing the oxidation of the surface.For example, the vacuum is pulled as low as possible, less than about 1Torr and preferably to 1 milli-Torr or lower. Then hydrogen gas isintroduced into the working zone at a partial pressure of 0.0005-10Torr, preferably at a partial pressure of 0.05-1.0 Torr, and mostpreferably at a pressure of less than about 0.001 Torr, the pressurebeing maintained.

After the preselected carburizing temperature is reached, the protectiveatmosphere is removed by stopping the flow of the protective gases.Optionally, as previously noted, the carburizing gas can be introducedat lower temperatures in conjunction with hydrogen or as a replacementfor hydrogen as its partial pressure is reduced, provided that it isadded at a temperature and in a volume so as not to form soot on thearticle surface. The carburizing temperature is in the range of1800°-2250° F., preferably 1800°-2100° F., more preferably 1900°-2050°F., and most preferably in the range of 1925°-2000° F. Carburizing gasis then introduced into the working zone of the furnace. Hydrocarbonshaving triple bonded carbon atoms, generally known as alkynes, thesimplest being acetylene (also known as ethyne) and represented by thechemical formula C₂H₂, H—C≡C—H, ethylene (C₂H₄) and propane arepreferred and are believed to be the most effective carburizing agentfor carburizing nickel-based superalloys that include refractoryelements in order to prevent the formation of TCP phases near thesurface of the substrate after aluminizing the article substrate, whenthe carburizing is performed under carefully controlled conditions ofthe present invention. Prior art methods that utilize chemical vapordeposition (CVD) techniques are time consuming and very limited by thesize of the chambers used for the CVD process. While larger CVD chamberscan be developed, this equipment is very expensive. The alkynes andethylene are more reactive and chemically unstable at the temperaturesof carburization of nickel-based superalloys than other carbon gases,such as saturated hydrocarbons including such widely used gases asmethane (CH₄) and propane (C₂H₆). Thus the alkynes, in particularacetylene, and ethylene, decompose into their constituent elements morereadily than the saturated hydrocarbons, thereby making free carbonreadily available at the substrate surface in the working zone of thefurnace. Because of their reactivity, care must be taken in thecarburizing process to prevent the introduction of oxidizing agents,such as oxygen, as an explosive mixture can result.

The carburizing gases may be introduced into the carburizing device byany method that prevents the introduction of oxygen. The preferredmethods are continuously flowing methods and pulse methods.

In the continuously flowing method, carburizing gas was introduced intothe working zone of the furnace at the preselected elevated carburizingtemperature. In this method, the carburizing gas was introduced at apartial pressure of about 2-3 Torr and this partial pressure wasmaintained for the duration of the carburizing operation, which was inthe range of about 1 to about 240 minutes, but preferably is about 10minutes. The preferred carburizing gas was acetylene with a carburizingtime in the range of 1-20 minutes. The carburizing time will varydepending upon the reactivity of the carburizing gas mixture and thetemperature of operation.

In the pulse method, which is most effective in a vacuum furnace, aswill become obvious, a pulse of carburizing gas is introduced into thefurnace at a preselected flow rate or to achieve a preselected partialpressure, for example, in the range of 0.1-10 Torr. The gas supply isthen closed to prevent any additional flow of carburizing gas. After aperiod of time, which will vary depending upon a number of factorsincluding but not limited to size of the working zone, size of the workload, temperature, vacuum pressure, etc. the working zone will bedepleted of carburizing gas and hence of carbon. At this point,additional carburizing gas is introduced into the furnace and theprocess is repeated. The period of time required will vary as previouslynoted, but a typical period is about 5 minutes. The process is repeateduntil carburization is completed. Thus, if it takes 10 minutes ofcarburization to achieve the desired depth, it is expected that twopulse cycles will be required.

Carburization is continued until the desired carburization depth isreached at which time the operation is stopped by introducing an inertgas at about 1800° F. to cool the article rapidly. Carburization ceaseswhen the article passes a critical temperature of less than 1800° F. Thedesired or target carburization depth is approximately equal to thedepth that aluminum penetrates below the substrate during thealuminizing process. Small deviations (a few microns), either slightlygreater than or slightly less than the target depth will not seriouslyimpact the properties of the article. Because the carburizing process isperformed before the aluminizing process, it is necessary to estimatethe depth of penetration of the aluminum. Of course, the depth ofpenetration of aluminum also will vary depending on a number of factorssuch as activity of aluminum, whether the process is thermally growninto the surface or an additive layer, processing temperature, thealuminizing process itself. However, experience indicates that therequired carburizing depth is between about 10 microns to about 100microns.

As will be recognized by those skilled in the art, several operatingparameters can be varied, therefore these parameters must be controlledto control the desired carbide layer thickness. These parametersinclude, but are not limited to gas flow rate, which determines partialgas pressure, temperature, type of furnace, working zone size, work loadand time. Gas flow rates of acetylene of about 100 liters/per hour forthe current furnace have been found to be acceptable, with flow ratesfrom as low as 50 liters/per hour to 100 liters per hour also likely tobe acceptable. Of course, flow rates will vary with furnace type,furnace size and work load.

After processing and cooling, the work load, which typically willcomprise a plurality of articles, can be removed from the work zone. Anyoptional masking may be removed before or after the aluminide treatment,depending upon whether or not the masked areas require aluminizing.Masking may be removed by any suitable means that does not adverselyaffect the substrate surface, such as chemical stripping, mechanicalmeans such as blasting, or other known methods consistent with themasking material. The articles may also be heat treated as required,either before or after aluminizing, to age or otherwise develop thefinal desired microstructure. These aging treatments are related toprecipitation hardening mechanisms of nickel-based superalloys, and havelittle or no effect on the stable carbide particles.

FIG. 5 illustrates the microstructure of the near-surface region of theblade 10 when the approach of the invention, just described in relationto FIG. 4, is followed. The structure is similar to that of FIG. 3, butno TCP phase is present and therefore no secondary reaction zone ispresent. Instead, an array of fine carbon-rich precipitates (carbides)36 are present in the region to which the deposited carbon atoms havediffused in sufficient quantity to form carbides. These carbidestypically contain refractory elements, such as rhenium, chromium,tantalum, tungsten, molybdenum, ruthenium, iridium, osmium and incertain alloys, palladium, rhodium and platinum, reducing the amount ofthese elements available to react to form the TCP phase in a depletedregion 38, which may equivalently be described as a carbide-precipitateregion. The carbides typically form within the gamma phase channels andare typically equal to or less than the size of the gamma primeprecipitates, less than 1 micron diameter. The term “depleted region”means that the concentration of TCP phase-forming elements in a formsuitable for reacting to form the TCP phase is reduced. The term shouldnot be taken to mean that those elements have been completely removedfrom the depleted region 38. Instead, the TCP phase forming refractoryelements are present, but in a substantially reacted form such that theycannot form the TCP phase.

As the article is aluminided, aluminum diffuses from the layer 20 intothe substrate to an extent indicated by an aluminide depth 40. Thisdiffusion, like the diffusion of carbon during carburization, isgoverned by the well known Fick's Second Law of Diffusion, beingdependent on time and temperature. The depleted region 38 extends to adepth which is preferably approximately equal to the aluminide depth 40,but may be slightly greater than or less than the aluminide depth 40.The depleted region 38 extends to a depth of from about 10 to about 100microns, preferably 25-100 microns below the surface of the substrate,and the aluminide layer 40 extends to a depth of from about 10 to about50 micrometers below the surface of the substrate. If the depletedregion 38 is substantially greater than the aluminide depth 40, theexcess volume of material is unnecessarily depleted of thesolid-solution strengthening refractory elements and includesunnecessary carbide precipitates. The carbide precipitates may causepremature failure of the superalloy if the depth of the region 38 is toogreat. If the depleted region 38 is substantially less than thealuminide depth 40, there will be a small region where the TCP phase mayform. The result is a secondary reaction zone that is smaller than wouldotherwise be present, but its presence is still detrimental.

Even though the near surface portion of the article includes carbides,typically tantalum carbides, that increase the hardness from about 40-45Rc to 55-60 Rc, these nickel-based superalloy articles can still besubject to traditional manufacturing processes such as drilling,coating, shot peening etc. Carburization does not inhibit suchtraditional manufacturing processes.

EXAMPLE 1

Articles were carburized in a Turbotreater® horizontal vacuumcarburizing furnace, Model H3636 AvaC™ Ipsen International Furnacehaving multiple nozzles for introducing gases. Such a furnace isavailable from Ipsen International of Rockford, Ill. The furnace has aworking zone of 3′×2′×2′ (l×w×h). The furnace utilizes carbon heatingelements that do not react with the gases introduced. The working zonewas loaded with a plurality of turbine blades, after cleaning, about10-50. These turbine blades are small commercial engine blades made ofrefractory-containing superalloy and having a size (overall bladelength) of about 1.5″. The blades were maintained under a reducingatmosphere, as hydrogen was introduced into the furnace to a pressure ofabout 0.150 Torr until the carburizing temperature of 1975° F. wasreached. Once at 1975° F., the hydrogen was evacuated from the furnacework zone and acetylene gas was then introduced into the furnace at aflow rate of approximately 100 liters per hour to maintain a pressure ofabout 2 Torr for a time of about 10 minutes. After the 10 minutes ofcarburization, the acetylene was evacuated from the furnace work zoneand nitrogen gas was introduced to allow for rapid cooling of the loadbelow about 1800° F. A zone of submicron carbide particles was formed inthe near surface region of the blades, the depth of which was about 66microns for the blades observed. A carburized blade was coated withplatinum modified beta nickel-aluminide coating and then exposed to2000° F. for about 400 hours. The aluminide coated blade after thermalexposure showed no formation of SRZ where an aluminide coatednon-carburized control sample produced about 0.004″ depth of SRZ thatcovered greater than 50% of the surface area.

EXAMPLE 2

A Turbotreater® horizontal vacuum carburizing furnace Model H3636 AvaC™Ipsen International Furnace was used, as described in Example 1. 1″diameter by 0.125″ thick specimen were maintained under a vacuumatmosphere of less than 0.001 Torr until the carburizing temperature of1975° F. was reached. Once the temperature was stabilized at 1975° F.,acetylene gas was introduced into the furnace at a flow rate of about100 liters per hour to maintain a pressure of about 2 Torr for a time ofabout 10 minutes. After about 10 minutes of carburization, the acetylenewas evacuated from the furnace work zone and argon gas was introduced toallow for rapid cooling of the furnace load below about 1800° F. A zoneof submicron carbide particles was formed in the near surface region ofthe blades, the depth of which was about 74 microns for the samples. Asample was coated with a platinum modified beta nickel-aluminide coatingant then exposed to 2000° F. for about 400 hours. The aluminide coatedspecimen after thermal exposure showed no formation of SRZ.

The present invention provides an improved structure to nickel-basesuperalloys that would otherwise be susceptible to formation of asecondary reaction zone. Such superalloys with aluminide, platinumaluminide (or other noble metal aluminide), and overlay coatings,including MCrAlY overlay coatings or beta nickel aluminide overlaycoatings such as disclosed in U.S. Pat. No. 6,261,084, incorporatedherein by reference, benefit from the approach of the invention. Theprocess used to form the stable carbides can be performed using thepreferred temperatures and carburization gases more quickly than otherknown methods. Despite the shorter times required, the precise depths ofcarbide formation can still be carefully controlled.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments.

1. A nickel based superalloy article, comprising: a nickel-basedsuperalloy substrate having a surface; refractory carbide precipitatesformed by carburizing to a preselected depth below the surface of atleast a portion of the superalloy substrate, the remainder of thesuperalloy substrate being substantially free of refractory carbideprecipitates from carburizing at or below the surface; and a diffusionaluminide coating formed on that portion of the substrate surface havingcarbide precipitates extending a preselected distance below the surface,the diffusion aluminide extending below the substrate surface to adistance substantially no greater than that of the refractory carbides.2. The nickel-based superalloy article of claim 1 wherein thenickel-based superalloy substrate is a turbine airfoil selected from thegroup consisting of blades and vanes.
 3. The superalloy article of claim1 wherein the refractory carbide precipitates are formed to apreselected depth below the substrate surface of between about 10-100microns.
 4. The superalloy article of claim 3 wherein the diffusionaluminide coating is formed to a distance of 10-50 microns below thesubstrate surface and wherein the distance is of the diffusion aluminidecoating extends to substantially the same distance below the substratesurface as the refractory carbide precipitates.
 5. The superalloyarticle of claim 1 further characterized by a substantial absence of TCPphases in a near-surface region.
 6. An article comprising: a substratecomprising a nickel-based superalloy, the substrate having a substratesurface; a carbide precipitate region comprising refractory carbideprecipitates, at least some of the refractory carbide precipitates beingdiffused into the substrate to a depth of about 10 to about 100micrometers below the substrate surface, the carbide precipitate regionbeing substantially free of acicular topologically close-packed (TCP)phase forms; an aluminum-based layer; and an aluminum-based diffusionregion contiguous to the aluminum-based layer, the aluminum-baseddiffusion region extending from the aluminum-based layer into thecarbide precipitate region, and into the substrate to a depth of about25 to about 50 micrometers below the substrate surface.
 7. The articleof claim 6, wherein the aluminum-based layer is contiguous to thecarbide precipitate region.
 8. The article of claim 6, wherein thealuminum-based layer is the outermost layer.
 9. The article of claim 6,wherein the at least some of the refractory carbide precipitatesdiffused into the substrate are diffused therein to a depth of about 25to about 100 micrometers below the substrate surface.
 10. The article ofclaim 6, wherein the article is a turbine airfoil blade.
 11. The articleof claim 6, wherein the article is a turbine airfoil vane.